Sonic energy is used in semiconductor fabrication to enhance wet chemical processing. The efficient transfer of energy between energy source and liquid chemical bath is limited, however, where the transducer must be protected from aggressive chemicals. Significant energy losses occur at interfaces, particularly where there is an air gap.
Some prior art systems have had a sonic transducer bonded to a metallic plate which in turn is mounted to an outside wall of a chemically inert polymeric vessel. The transducer can be bonded firmly to the metal plate and the plate provides means for removing heat from the transducer, improving its reliability. But this structure has not been adequate to provide efficient delivery of sonic energy to the chemical bath where it is desired to form vessel walls from a flourocarbon polymer, such as teflon, for which there is no good adhesive between metal plate and polymer wall. The inability to provide intimate bonding at this interface has limited the use of such polymers for tank walls.
To counter this problem, other systems have sealed sonic transducers in a polymeric protective coating and placed them inside the processing vessel. However, this solution exposed the polymer coating, transducers and their electrical connections to the sometimes corrosive properties of the processing liquid. Polymer coating or seal failures have sometimes led to the contamination of semiconductor wafers in the chemical bath or to failure of the transducers or their mountings.
Because the thermal expansion coefficient of polymers is usually much larger than that of the transducers and substrates, where there is heating of the transducer or substrate, differential thermal expansion increases the gap between transducer or substrate and the coating, reducing the efficiency of energy transfer. This in turn increases the local temperature, exacerbating the problem and lowering transducer reliability. The thermal expansion not only degrades coupling of energy between transducer and processing fluid, it also can cause stress cracks and leaks through the polymer.
The air gap problem is best illustrated for an immersion heater type transducer that has a polymer coating. If an air gap develops between heater and coating the flow of heat out of the heater will be reduced, and so the temperature of the heater will increase. Portions of the polymer coating still contacting the heater will also get hotter. Since the polymer coating typically has a much larger thermal expansion coefficient than the heater, the polymer coating is likely to further expand away from the heater. In this case the heater will continue to increase in temperature, and ultimately the polymer will melt at the few points of contact.
A better solution is needed that provides a high level of coupling between transducers and their polymeric protective coats and a high level of transmission through the protective coats to the processing liquid chemical bath without introducing the risk of seal failures, and this solution is provided by the following invention.